Spring源码讲解之 getBean 方法

getBean 方法是用来进行获取 bean 实例对象的，是我们进行 spring 程序开发从 bean 工厂获取结果的。那这个方法的执行过程是什么样的，下面我们就去揭开它的面纱。

样例代码

ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
User user = ctx.getBean("user", User.class);

getBean 方法执行过程

org.springframework.context.support.AbstractApplicationContext.getBean(String, Class)

@Override
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {// 判断 bean 工厂是否处于激活状态// 对于 ClassPathXmlApplicationContext 实例此方法默认实现是空的assertBeanFactoryActive();return getBeanFactory().getBean(name, requiredType);
}

org.springframework.context.support.AbstractRefreshableApplicationContext.getBeanFactory()

@Override
public final ConfigurableListableBeanFactory getBeanFactory() {DefaultListableBeanFactory beanFactory = this.beanFactory;if (beanFactory == null) {throw new IllegalStateException("BeanFactory not initialized or already closed - " +"call 'refresh' before accessing beans via the ApplicationContext");}return beanFactory;
}

org.springframework.beans.factory.support.AbstractBeanFactory.getBean(String, Class)

@Overridepublic <T> T getBean(String name, Class<T> requiredType) throws BeansException {return doGetBean(name, requiredType, null, false);}

org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(String, Class, Object[], boolean)

@SuppressWarnings("unchecked")
protected <T> T doGetBean(String name, @Nullable Class<T> requiredType,@Nullable Object[] args, boolean typeCheckOnly) throws BeansException {// bean 名称处理：去除工厂引用前缀，规范别名规则String beanName = transformedBeanName(name);Object bean;// 首先，从单例缓存中获取 beanName 对应的 bean 实例// 如果单例缓存中没有则会新建.Object sharedInstance = getSingleton(beanName);if (sharedInstance != null && args == null) {if (logger.isTraceEnabled()) {if (isSingletonCurrentlyInCreation(beanName)) {logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +"' that is not fully initialized yet - a consequence of a circular reference");} else {logger.trace("Returning cached instance of singleton bean '" + beanName + "'");}}// 注入该实例需要依赖的 bean bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);} else {// Fail if we're already creating this bean instance:// We're assumably within a circular reference.if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// Check if bean definition exists in this factory.BeanFactory parentBeanFactory = getParentBeanFactory();if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// Not found -> check parent.String nameToLookup = originalBeanName(name);if (parentBeanFactory instanceof AbstractBeanFactory) {return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);} else if (args != null) {// Delegation to parent with explicit args.return (T) parentBeanFactory.getBean(nameToLookup, args);} else if (requiredType != null) {// No args -> delegate to standard getBean method.return parentBeanFactory.getBean(nameToLookup, requiredType);} else {return (T) parentBeanFactory.getBean(nameToLookup);}}if (!typeCheckOnly) {markBeanAsCreated(beanName);}try {RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.String[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dep : dependsOn) {if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");}registerDependentBean(dep, beanName);try {getBean(dep);}catch (NoSuchBeanDefinitionException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"'" + beanName + "' depends on missing bean '" + dep + "'", ex);}}}// Create bean instance.if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that received a temporary reference to the bean.destroySingleton(beanName);throw ex;}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}else if (mbd.isPrototype()) {// It's a prototype -> create a new instance.Object prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}else {String scopeName = mbd.getScope();if (!StringUtils.hasLength(scopeName)) {throw new IllegalStateException("No scope name defined for bean 麓" + beanName + "'");}Scope scope = this.scopes.get(scopeName);if (scope == null) {throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");}try {Object scopedInstance = scope.get(beanName, () -> {beforePrototypeCreation(beanName);try {return createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}});bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);}catch (IllegalStateException ex) {throw new BeanCreationException(beanName,"Scope '" + scopeName + "' is not active for the current thread; consider " +"defining a scoped proxy for this bean if you intend to refer to it from a singleton",ex);}}}catch (BeansException ex) {cleanupAfterBeanCreationFailure(beanName);throw ex;}}// 检查 bean 实例类型是否与指定的类型相匹配.if (requiredType != null && !requiredType.isInstance(bean)) {try {T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);if (convertedBean == null) {throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());}return convertedBean;} catch (TypeMismatchException ex) {if (logger.isTraceEnabled()) {logger.trace("Failed to convert bean '" + name + "' to required type '" +ClassUtils.getQualifiedName(requiredType) + "'", ex);}throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());}}return (T) bean;
}

至此 getBean 方法的执行过程已讲解完毕。可能大家注意到 doGetBean 部分代码没有讲解到，这是由于咱们的示例代码在执行的过程中没有执行到这部分的代码，所以没有给大家讲解，后续如果样例程序中涉及到这部分代码，我会详细讲解的。读过 spring 源码的朋友都知道，spring 的架构非常好，考虑非常周全，而且扩展性特别强，这是值得我们学习和借鉴的。

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Spring源码讲解之 getBean 方法

样例代码

getBean 方法执行过程

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